II.] THE HEAD-FOLD, 35 



form of the letter 8. This fold we shall always speak of 

 as the head-fold. In it we may recognize two limbs: 

 an upper limb in which the curve is directed forwards, 

 and its bay, opening backwards, is underneath the blas- 

 toderm, i.e. as we shall see, inside the embryo (Fig. 10. 

 D) ; and an under limb in which the curve is directed 

 backwards, and its bay, opening forwards, is above the 

 blastoderm, i.e. outside the embryo. If an 8 like the above, 

 made of some elastic material, were stretched laterall}^ 

 the effect would be to make both limbs longer and 

 proportionally narrower, and their bays, instead of being 

 shallow cups, would become more tubular. Such a 

 result is in part arrived at by the growth of the blasto- 

 derm; the upper limb of the g is continually growing 

 forward (but, unlike the stretched elastic model, in- 

 creases in all its dimensions at the same time), and the 

 lower limb is as continually lengthening backwards; 

 and thus both upper and lower bays become longer and 

 longer. This we shall hereafter speak of as the travel- 

 ling backwards of the head-fold. 



The two bays do not however both become tubular. 

 The section we have been speaking of is supposed to be 

 taken vertically along a line, which will afterwards be- 

 come the axis of the embryo; and the lower bay of the 

 8 is a section of the crescentic groove mentioned above, 

 in its middle or deepest part. On either side of the 

 middle line the groove gradually becomes shallower. 

 Hence in sections taken on either side of the middle 

 line or axis of the embryo (above or below the plane 

 of the figures), the groove would appear the less marked 

 the farther the section from the middle line, and at a 

 certain distance would disappear altogether. It must be 



32 



